It is well known that certain transistorized logic circuits are used extensively in digital computing, communications, control and data handling equipment as well as in other electronic systems and apparatus. For example, the presently disclosed solid-state logic circuit finds particular utility in a vital type of automatic vehicle speed control system for railroad and mass and/or rapid transit operations. In vehicular speed control systems of this type, it is of vital and utmost importance, and in many cases an authoritative requirement, to ensure that certain circuit sections or portions of the control system must operate in a fail-safe fashion. It will be appreciated that in fail-safe operation, a circuit malfunction or component failure must result either in a more restrictive condition or in a condition which is as safe as that preceding the failure.
While certain nonvital static gates, such as, "OR" logic circuits, are well known in the art, it is readily apparent that these conventional logic circuits are possessed of certain shortcomings which make their direct application to an automatic vehicle speed control system normally unacceptable and generally unsuitable for railroad and transit operation. For example, these standard nonvital solid-state "OR" gates operate in an unsafe manner in that a false output can be produced when a component or circuit failure occurs. The opening or short circuiting of an inductor, capacitor, resistor, diode, or transistor or the crossing or the loss of a lead or conductor can result in an erroneous output in an ordinary and conventional solid-state "OR" logic circuit. Further, a change or variation in the resistive, capacitive or inductive value of a component or element can cause a fluctuation in the amplitude of an output signal and can result in spurious signals. In one instance, it was found to be advantageous to utilize a plurality of vital level detectors of the oscillating type to make up an "OR" logic gate for a vehicle speed control system to determine if either of two inputs was present in the required amount. However, upon further experimentation, it was discovered that a problem occurred when two or more oscillating level detectors were separately or individually coupled to an amplifier gate. If both of the oscillating level detectors are turned on or rendered conductive, any difference in the frequencies of oscillations results in the production of a beat frequency. If the output of the "OR" gate is employed in a certain application, such as, in a vital vehicle speed control system, the introduction of the resultant low frequency beat signal into the system is wholly unacceptable and can cause an unsafe condition. Thus, it is mandatory in a fail-safe or vital speed control system to ensure that a beat frequency is not produced by a logic circuit employing a plurality of oscillating level detectors. Accordingly, it is necessary to ensure that unwanted beat signals are not produced when both oscillators are rendered conductive during the presence of both d.c. input signals.